Priority of my GB Patent Application 0006709.0, filed Mar. 20, 2000, incorporated herein by reference, is hereby claimed.
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1. Field of the Invention
The apparatus of the present invention relates to ways of making a flat-panel display by projecting the image from a microdisplay.
2. General Background of the Invention
Flat-panel displays which have screens large enough to stimulate the quick reactions of our peripheral vision will give pictures great immediacy, yet because they are flat the displays will fit easily onto the wall of a room. The size of conventional flat-panel displays however is limited by the resistor/capacitor time constant of the row and column transparent conductors, and by the area over which lithography can be sufficiently precise to make transistors. The cost of making active-matrix liquid-crystal displays with screen diagonals greater than one meter is prohibitive, and even the cheaper plasma displays are too expensive for most uses. However, costs decrease considerably with size and the 2xe2x80x3 by 2xe2x80x3 (5 cmxc3x975 cm) liquid crystal displays used in video projectors are relatively cheap, while fingernail-sized microdisplays look set to cost only a few dollars.
Video projectors comprise a two-dimensional display, a projection lens and a translucent screen. The projection lens forms on the translucent screen a magnified image of the two-dimensional display which can be almost as big as one wants. Video projectors are cheap, so are becoming increasingly popular, but often the projector gets in the way of the viewer, or the viewer gets in the way of the projected light. Furthermore, unless the room lights are dimmed, the image looks washed out because the screen scatters background light as well as the projected image.
A group of video projectors is able to project a three-dimensional image if they are all put in the focal plane of a screen-sized lens. Each projector is positioned to form a picture in the plane of the field lens just as if the lens were a translucent screen, but unlike a translucent screen the field lens collimates the light so that the picture is visible from only a single direction. The other projectors form views which are made visible by the field lens to other directions so that the viewer sees an autostereoscopic three-dimensional image. In this application too, however, the projected light is easily blocked. What is needed is a display which is as cheap as a video projector, but is a slim, integrated device like a flat-panel display.
A display capable of expanding an image from a small projector into a large viewable plane is shown in U.S. Pat. No. 5,381,502 (James T. Veligdan). The plane is formed by a stack of thin sheet waveguides the edges of which on one side form the lines of the display. The stack is tapered to form a thin wedge, and each line of the image is injected into the appropriate sheet at the blunt end of the wedge, to emerge from the tapered face of the plane. The manufacturing of such a stack is not straightforward. Applicant is submitting herewith an Information Disclosure Statement of prior art patents, which applicant asks that they be incorporated herein by reference thereto.
According to an aspect of the present invention there is provided a waveguide display or light modulator comprising a video projector arranged to emit modulated light rays generally along a direction of propagation, over a range of angles relative to that direction; and a waveguide extending along the axis of propagation and arranged to receive rays injected from the projector and to eject rays at a distance along the axis of propagation along the waveguide that depends on their angle of injection. Preferably means are also provided for magnifying the projected image in the transverse direction before it enters the waveguide; the waveguide can then take the form of a large display panel.
A simple way of making a waveguide eject the rays at a certain distance along its length (i.e. along the direction of the propagation of the rays within the waveguide) is to have the waveguide in a slab shape preferably of a uniform material but tapering along its length. This means that the angle of incidence of the trapped rays on the surface of the slab becomes ever steeper until waveguiding is no longer possible and the ray, or a portion of it, escapes from the surface of the waveguide, a process which can be assisted using suitable antireflection coatings. It can then be directed suitably towards the viewer. Of course, the steeper the initial angle of the rays into the waveguide, the sooner they escape, which means that the steepest injected rays are those that make the image over that part of the waveguide nearest the projector, and the flattest (nearest parallel) rays form the furthest parts of the image. This principle is known from, for instance, EP-A1-0663600 (Nitto Jushi Kogyo K.K.).
The magnifier can be itself a slab waveguide, positioned edge-on between the projector and the ejecting waveguide, and having the same width as the screen (i.e. of the ejecting waveguide) so as to allow the rays to spread out in the transverse direction but (assuming that the sides of the magnifying waveguide are parallel) to conserve the out-of-plane angle of the rays. The system can be folded over so that the tapered waveguide and the magnifying waveguide overlap.
As an alternative to the slab waveguide magnifier an essentially one-dimensional input waveguide can be used, which tapers transversely to the ejecting waveguide, being arranged along the input edge of the latter waveguide in such a way that light input at the thick end of the input waveguide is ejected along its length into the ejecting waveguide.
In an alternative aspect the invention is directed to a planar light source comprising an ejecting waveguide as aforesaid and a one- or zero-dimensional light source whose output is fed into the waveguide so as to emerge over the plane of the waveguide. This represents a simple way of providing a planar, in particular a collimated, light source such as is useful for displays.
The invention also envisages a large-area display in which a small-area image source is arranged to project an image into a slab waveguide much wider than the source, so that the image is contained in the plane of the waveguide but expands transversely, and emerges from the waveguide to be projected on to a screen arranged parallel but offset to the slab. Here the two-dimensional spread of the image is achieved on the one hand, as before, by the transverse spread within the waveguide and on the other hand, differing from the previous method, by allowing the light emerging from the end of the waveguide, still essentially collimated, to strike the screen obliquely.
For a better understanding of the invention embodiments of it will now be described, by way of example, with reference to the accompanying drawings.